Heretofore, one set of a roughing cutter, 1 as shown in FIG. 5, and a finishing cutter 2, as shown in FIG. 6, has been employed to countersink holes defined in FRP materials such as using CFRP, GFRP or KFRP. In such FIGS., reference character W denotes a workpiece which is made of any of the above FRP's and is to be machined, and H denotes a hole which is defined in the workpiece W and is to be countersunk.
In the roughing cutter 1 shown in FIG. 5, a cutter body 3 is attached to an air drilling tool, an electric-powered tool, or the like, for rotation about its axis O, and a pilot portion 4 is formed to project from a distal end of the cutter body 3 along the axis O. The pilot portion 4 has an outer diameter sized so that it may be inserted into the hole H defined in the workpiece W.
The cutter body 3 is provided, in a region closer to the base end than the pilot portion 4, with conical surfaces 5 flaring toward the base end side radially outwardly with respect to the axis O and the chip discharge grooves 6, radially recessed from the conical surfaces 5, such surfaces 5 and grooves 6 being alternately formed in the circumferential direction of the cutter body 3. Each of the conical surfaces 5 has an abrasive layer 7 formed thereon by electrodepositing diamond abrasive grains.
As with the above-described roughing cutter 1, the finishing cutter 2, shown in FIG. 6, includes a pilot portion 9 formed at a distal end of a cutter body 8, conical surfaces 10 and chip discharge grooves 11 formed in a region closer to the base end than the pilot portion 9.
Further, in the finishing cutter 2, a hard sinter 12, made of diamond as a main ingredient, and a hard metal 13 are laminated into a laminar sinter 14 in the form a fan-shaped flat plate. The laminar sinter 14 is brazed to a wall surface 11A of the chip discharge groove 11 and facing in the direction of rotation of the tool such that the hard sinter 12 faces in the direction of rotation of the tool. A finishing edge 15 is formed on the hard sinter 12 along a ridge where the wall surface 11A and the conical surface 10 join with each other.
When countersinking the hole H by the cutters 1, 2, the roughing cutter 1 is first attached to the air drilling tool, the electric-powered tool, or the like, not shown, followed by presetting, such as, adjustment of a depth by which the hole H is to be countersunk. Then, the axis O of the cutter body 3 is aligned with an axis of the hole H, and the cutter body 3 is rotated and fed into the hole H while being guided with the aid of the pilot portion 4. A part of the workpiece W, around the hole H, is gradually cut by the abrasive layers 7 formed on the conical surfaces 5. As a result, a roughly cut surface R in the form of a recessed conical surface diverging toward the center of the hole H, is formed.
After the roughing step, the roughing cutter 1 is removed from the air drilling tool, or the like, and the finishing cutter 2 is attached to the tool and presetting to the depth by which the hole is to be countersunk. Then, similarly to the above step, the cutter body 8 is rotated and fed into the hole H while being guided with the aid of the pilot portion 9. As a result, the roughly cut surface R is shaven by the finishing edge 15 to form a smooth finished surface F.
The countersinking by the use of roughing cutter 1 and the finishing cutter 2 comprises two steps, i.e., the roughing step and the finishing step. It is impossible to make a machining time required to complete the entire countersinking shorter than the total time necessary for the two machining steps. This has been one impediment to improvement in the machining efficiency. Further, when the machining is shifted from the roughing step to the finishing step, it is required to remove the roughing cutter 1 from the air drilling tool, or the like, attach the finishing cutter 2 to the tool, and preform the presetting such as adjustment of the depth by which the hole is to be countersunk, before starting the roughing step. This has inevitably resulted in longer machining time.
Also, because one cutter must be replaced with the other cutter upon shift from the roughing step to the finishing step, frequent replacement of the cutters has imposed a great deal of labor on an operator, particularly when there are many holes H to be countersunk. Additionally, the roughing cutter 1 and the finishing cutter 2 must be handled separately and the total number of cutters is increased, resulting in still another problem and making tool management complicated.